On Fri, Dec 14, 2012 at 6:17 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > This is a serious cause of mmap_sem contention. MAP_POPULATE > and MCL_FUTURE, in particular, are disastrous in multithreaded programs. > > Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx> > --- > > Changes from v1: > > The non-unlocking versions of do_mmap_pgoff and mmap_region are still > available for aio_setup_ring's benefit. In theory, aio_setup_ring > would do better with a lock-downgrading version, but that would be > somewhat ugly and doesn't help my workload. Hi Andy, I agree that the long mmap_sem hold times when using MAP_POPULATE, MAP_LOCKED or MCL_FUTURE are a problem. However, I'm not entirely happy with your proposed solution. My main concern is that just downgrading the mmap_sem only hides the problem: as soon as a writer gets queued on that mmap_sem, reader/writer fairness kicks in and blocks any new readers, which makes the problem reappear. So in order to completely fix the issue, we should look for a way that doesn't require holding the mmap_sem (even in read mode) for the entire duration of the populate or mlock operation. I think this could be done by extending the mlock work I did as part of v2.6.38-rc1. The commit message for fed067da46ad3b9acedaf794a5f05d0bc153280b explains the idea; basically mlock() was split into do_mlock() which just sets the VM_LOCKED flag on vmas as needed, and do_mlock_pages() which goes through a range of addresses and actually populates/mlocks each individual page that is part of a VM_LOCKED vma. This could be easily extended for mlocks that happen in mmap_region() due to MAP_LOCKED or MCL_FUTURE: mmap_region() would just set the VM_LOCKED flag and defer the work of actually populating/mlocking the individual pages. I think the only constraint here is that the pages must be locked before returning to userspace, so we may be able to use the task_work mechanism to achieve that. Later on (but before returning to userspace) we would notice we have some mlock work to do and call do_mlock_pages() to achieve that. I think the benefits of this approach would be: - no mmap_sem locking changes around mmap_region() - which also means that all code paths to mmap_region() can instantly benefit - do_mlock_pages() doesn't need to hold a read lock on mmap_sem for the entire duration of the operation, so we can fully solve the problem instead of just making it harder to trigger Now for handling MAP_POPULATE, we would probably want to use a similar mechanism as well, so that we don't need to hold the mmap_sem for the entire duration of the populate. This is similar in principle to the MAP_LOCKED case; however this may require the introduction of a new VM_POPULATE vma flag in order to avoid the possibility of a race where someone replaces our vma with another before we get a chance to populate it. I don't have an implementation for this idea yet; however I'm hoping to come up with one before xmas. Before then, any comments on the idea ? -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>